With the ever-changing technology, more portable electronic devices such as mobile phone, digital camera, tablet computer and notebook computer have become a necessary tool for people's lives. Since a portable electronic device is generally used by an individual and it has a certain property of privacy, the data such as a telephone book, a photograph, personal information and the like stored therein is privately owned. If the electronic device is lost, the data may be used by others, resulting in unnecessary losses. While a manner of password protection has been used at present to avoid that the electronic device is used by others, a password is easy to be leaked or deciphered with lower security. Moreover, a user is required to remember the password when using an electronic device. If the password is forgotten, it will cause a lot of inconvenience. Therefore, a personal fingerprint identification system is developed currently to achieve identity authentication, so as to enhance data security.
In general, a capacitive sensing circuit is used to receive finger contact in a fingerprint identification system. The capacitive sensing circuit is used to accept finger contact and form a contact capacitor with a finger. The capacitive sensing circuit may convert a capacitance value of the contact capacitor into an analog voltage signal, which is converted into a digital voltage signal and then sent to a fingerprint determination module at a back end to determine finger ridges (Finger Ridge) or finger valleys (Finger Valley) of a fingerprint corresponding to the capacitive sensing circuit. In other words, the analog voltage signal is converted into the digital voltage signal through an analog-to-digital converter (Analog to Digital Convertor, ADC). However, a circuit structure of the analog-to-digital converter is complex and occupies a large circuit area, increasing production costs and consuming higher power. Therefore, providing a capacitive sensing circuit with simple circuit structure, small circuit area, low cost and low power consumption has become one of goals pursued in the industry.